It is sometimes desired that an automatic player piano having such a playback mode of operation be used as a background music player. When used for such a purpose, it is further desired that the player piano play back a music with utmost softness preferably through generation of sounds with a dynamic range even lower than the lowest dynamic range established for the piano. If a hammer head of the action mechanism of the player piano is driven with a force less than the force corresponding to such a minimum dynamic level, the hammer head could not effectively strike against the string and accordingly would fail to generate sound at the string. it may also be pointed out that a force reduced excessively could not drive the hammer head for movement at a proper velocity toward the string and would thus could not strike against the string at a proper timing, destroying the tempo at which the music is to be played back. It is a prime object of the present invention to provide an automatic player piano free from these problems encountered when the piano is to operate in playback mode at a lowered sound level.